Rinse aid compositions designed for use in automatic dishwasher machines are well known. These compositions are added during the rinsing cycle of the machine, separately from the detergent composition employed in the main wash cycle(s). The ability to enhance rinsing, and in particular the ability to prevent spot and film formation are common measures of rinse aid performance.
Rinse aid compositions typically contain components such as nonionic surfactants and/or hydrotropes which aid the wetting of the items in the rinse, thereby improving the efficacy of the rinsing process. These surfactants, and rinse aid compositions in general, are not designed for the achievement of a primary soil removal purpose.
The general problem of the formation of deposits as spots and films on the articles in the wash/rinse, and on the dishwasher machine parts is well known in the art.
Whilst the general problem of deposit formation is known, a full understanding of the many facets of the problem is however still an active area of research.
A range of deposit types can be encountered. The redoposition of soils or the breakdown products thereof, which have previously been removed from the soiled tableware in the washload, provides one deposit type. Insoluble salts such as calcium carbonate, calcium fatty acid salts (lime soaps), or certain silicate salts are other common deposit types. Composite deposit types are also common. Indeed, once an initial minor deposit forms it can act as a "seeding centre" for the formation of a larger, possibly composite, deposit structure.
Deposit formation can occur on a range of commonly encountered substrate surfaces including plastic, glass, metal and china surfaces. Certain deposit types however, show a greater propensity to deposit on certain substrates. For example, lime soap deposit formation tends to be a particular problem on plastic substrates.
The formation of insoluble carbonate, especially calcium carbonate, deposits is a particular problem in the machine dishwashing art. There is a general appreciation in the art, as represented for example by EP-A-364,067 in the name of Clorox, CH-A-673,033 in the name of Cosmina, and EP-A-551,670 in the name of Unilever, that calcium carbonate deposit formation is a particular problem when non-phosphate containing detergent formulations are employed. In general, this can be explained by the slightly inferior builder capacity of the typically employed non-phosphate builder systems in comparison to phosphate builder formulations. The problem of calcium carbonate deposit formation is understood to be especially apparent when these formulations contain a carbonate builder component, as for example is essential to the compositions taught by EP-A-364,067.
The Applicants have now found that the problem of CaCO.sub.3 deposit formation can exist even in the absence of a carbonate builder component in the machine dishwashing detergent formulation, and especially when that formulation contains no phosphate builder components. It has also been established that the problem is most apparent when highly alkaline formulations, such as those of pH of 9.8 and above, are employed.
The naturally sourced, inlet water to the dishwasher machine can be a sufficient source of Ca.sup.2+ and Mg.sup.2+ ions and CO.sub.3.sup.2- /HCO.sub.3.sup.- ions to make deposit formation a problem. Whilst the salt softening system, through which the inlet water will pass prior to entry into the main cavity of the dishwasher machine, can be efficient at removing the naturally present Ca.sup.2+ and Mg.sup.2+ ions it is inefficient at removing the CO.sub.3.sup.2- /HCO.sub.3.sup.- ions which therefore enter into the wash/rinse solution.
The Applicants have now established that both the levels of Ca.sup.2+ /Mg.sup.2+ hardness ions and the levels of CO.sub.3.sup.2- /HCO.sub.3.sup.- ions in the wash/rinse water of the dishwasher machine are factors controlling calcium carbonate deposit formation. Critical levels of both components must be exceeded for deposit formation to occur. These critical levels are to an extent interdependent. Thus, even in wash/rinse solutions containing high levels of one component, deposit formation will not occur in the absence of the critical level of the other component.
The Applicants have further established that the formation of calcium carbonate deposits occurs most noticeably in the rinse cycle of the dishwasher machine. Deposit build up is most apparent on the heater element of the dishwasher machine.
The Applicants have found that the problem of calcium carbonate deposit formation may be effectively ameliorated by the inclusion of an organo aminophosphonic acid component into a rinse aid formulation. Said rinse aid formulation is of particular utility when used in combination with non-phosphate containing detergent formulations which, as previously mentioned, tend to be more susceptible to the problem of calcium carbonate deposit formation.
The Applicants have also found that carboxylates and polycarboxylates, particularly citrates, are especially useful components of the compositions of the invention because of their magnesium binding capacity which tends to prevent the formation of insoluble magnesium salts, such as magnesium silicate on the articles in the wash. Such polycarboxylates also provide calcium binding capacity to the compositions, thus contributing further to the prevention of the formation of calcium salt deposits.
The Applicants have also found that the more effective control of calcium carbonate deposition can also lead to benefits in the prevention of the formation of other deposit types, particularly lime soap deposits and silicate deposits.
Lime soap deposits are most commonly encountered when the washload contains fatty soils, which naturally contain levels of free fatty acids, and when lipolytic enzymes are components of the formulation. Lipolytic enzymes catalyse the degradation of fatty soils into free fatty acids and glycerol. Silicate is a common component of machine dishwashing formulations, where it is added for its china care capability. It is the Applicant's finding that by preventing the formation of calcium carbonate deposit "seeding centres", the build up of other deposit types from these "seeding centres" is also prevented.
The Applicants have found that certain resistant soils/stains, especially bleachable soils/stains, most especially tea stains, can remain on tableware, especially chinaware at the end of the wash cycle of an automatic dishwashing machine.
The Applicants have also found that said resistant soils/stains, especially tea stains on chinaware, may `recolourise` under the conditions of the rinse, thereby enhancing the colour of the soils/stains.
The Applicants have found that the inclusion of said aminophosphonic acid component into said rinse aid formulation enhances the removal of said resistant soils/stains from the tableware during the rinse cycle. The problem of stain recolourisation is thus also avoided. The removal of tea stains from chinaware is particularly enhanced.